install Cuda 9.1 for my rtx 2070 card?

Can I install Cuda 9.1 on my Ubuntu 18.04 and will it be compatible with my card?
rtx 2070

But you are welcome to try.

But why I can’t use Cuda 9.1 and an RTX card?

NVIDIA recommends CUDA 10 or newer for RTX GPUs.

CUDA 10 has support for the sm_75 Turing architecture.

If you use a CUDA version prior to CUDA 10, you can still get those to work, but you may run into various issues such as long JIT compile delays if you use libraries like CUBLAS and CUDNN. And you won’t be able to compile codes specifically for the Turing architecture. You will need to compile for some previous architecture and make sure to use compile switches that will generate embedded PTX, for forward compatibility.